Heretofore, as a display device substitutable for a liquid crystal display (LCD), new display devices employing technologies such as an electrophoretic method, an electrochromic method, a thermal method, and a dichroic-particles-rotary method have been proposed. Since these technologies have the advantages described below, the applications thereof to the next generation image display devices, such as a display element for use as a personal digital assistance, or an electronic paper, have received attention. That is, the advantages include: excellent image quality close to regular printed matter since a wider viewing angle can be attained compared to a liquid crystal display, lower electrical power consumption, and the so-called “memory performance”, which is a characteristic of preserving images after the power is off.
Of these technologies, the image display technology employing the electrophoretic method is such that image display is achieved by disposing a solution containing dispersed particles in a colored solution between a pair of substrates facing each other, followed by applying voltage of about several tens of volts between the substrates, to result in migration of particles in the liquid phase. As an image display technology employing the electrophoretic method, there have been proposed technologies such that a dispersed solution is contained in microcapsules, which are then disposed between a pair of substrates facing each other (for example, refer to Non-Patent Document 1). This technology is considered to be the most feasible for a practical application. However, it has the problem that maintaining a displayed image circumstances is hard to realize.
Specifically, the problem is the difference in the specific gravity between a colored solution and dispersed particles, that is, when the difference in specific gravity between them becomes excessive, the dispersed particles tend to sediment in the colored solution, resulting in a problem of achieving stable image display. For example, when dispersed particles of a high specific gravity, such as titanium oxide, are used in a colored solution of a low specific gravity, the dispersed particles tends to precipitate in the colored solution. Further, the colored solution containing a dye usually shows poor storage stability, so that it has been difficult to maintain the image quality of the display device containing a dye.
On the other hand, an image display technology without use of solutions has been proposed. For example, there exists an image display technique based on a powder migration method, in which charged particles are enclosed in a gaseous phase, and, by applying voltage, the aforesaid particles are allowed to migrate along a direction of an electric field. According to this method the aforementioned problem concerning the image display device employing an electrophoretic method may be avoided. Also, by providing a difference in mobilities of the enclosed particles based on a difference in the polarities of the particles, an image display using plural kinds of particles can be carried out. For example, a sharp image having a high level of whiteness and visibility which have not been attained by an image display device employing the electrophoretic method can be obtained by using white particles and black particles and by providing different polarities to the two kinds of particles, whereby the mobilities of the white particles and the black particles are different.
Thus, the image display technique based on a powder migration method is expected to greatly contribute to improving the image quality of an image display device of the next generation such as electronic paper.
In the image display technique based on a powder migration method, the image display is carried out by enclosing charged particles between two opposing substrates and by driving the charged particles with an electric field applied between the substrates. Accordingly, the user observes the image through a substrate. Therefore, as the substrate constructing the display, materials having some extent of light transmittance are demanded in view of assuring visibility. Glasses represented by quartz glass, or organic-glasses such as polycarbonate have been employed.
Among these glasses, organic-glasses are effective to obtain a lightweight or a thin image display device. Recently, an image display device enabling flexible deformation has been obtained by employing a flexible substrate. Thus, the material for the substrate has a tendency to be selected according to the purpose of the use and the need for a resin substrate has become higher than ever (refer to, for example, Patent Document 1).
In the image display devise based on a powder migration method, since the image is observed through the substrate as abovementioned, desired is a material for the substrate exhibiting high transmittance of light as well as excellent durability, and a resin substrate has been employed in addition to a glass substrate. However, an image display device having a resin substrate has had a tendency that the image looks darker than the image of a display in which a glass substrate is used.
The reason is assumed to be that it is difficult for a resin substrate to transmit light evenly throughout the wavelength range of the visible light. In fact, in a polycarbonate substrate or a polyethylene terephthalate substrate, the light closer to the UV region tends to be more difficult to transmit than the light of other wavelength region. Alternatively, in an image display device employing a glass substrate, a bright image has been obtained since the glass substrate transmits more light than the resin substrate in the region closer to the UV region.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-248247
Non-Patent Document 1: Gugrae-Jo, et al., “New Toner Display Device (I)”, The Annual Conference of the Imaging Society of Japan, Jul. 21, 1999, “Japan Hard Copy '99”, pp. 249-252